The invention relates to a locating method for determining the location of a terminal in a radio system comprising a plurality of base stations and terminals.
The invention further relates to a locating arrangement for determining the location of a terminal in a radio system comprising a plurality of base stations and terminals.
A typical mobile telephone system covers a large geographical area and comprises a plurality of coverage areas, i.e. cells, each usually served by one base station. The cells vary greatly in size, usually in accordance with the amount of the call traffic in the area. In areas of high traffic density, the cell size is usually smaller than in areas where fewer calls are made. Irrespective of the size of a cell, it would often be useful to know the geographical location or direction of movement of a subscriber terminal.
Several different methods have been developed for determining the geographical location of terminals. The location information of a terminal can be utilized for a number of purposes. If the user desires, he or she can determine his or her location; the location of a terminal placing an emergency call can be determined in order to be able to send help; or call charging can be dependent on the location. The distance of the terminal from the base station communicating with it can easily be determined on the basis of propagation delay. Among the most widely used methods is a triangulation method in which the signal of the terminal is measured by means of three or more base stations, and the location of the terminal is computed on the basis of the propagation delay of the signals. This solution is also known as the TA (Timing Advance) method.
In another known solution, the terminal receives information on the clocks and location coordinates of the base stations located in the vicinity of the terminal from the cellular system. Next, the terminal measures the time differences of the signals received from the base stations in question and determines its location on the basis of the measured time differences and the information received from the system.
The problem of the known methods, such as the triangulation, is however that the location of the terminal cannot be found out very accurately and the location information thus obtained cannot be utilized in applications in which accurate geographical parameters are essential.
An object of the invention is thus to provide a method and an arrangement implementing the method so as to solve the above problems and determine the location of a terminal accurately.
This is achieved by a method of the type disclosed in the introduction, the method being characterized in that it comprises the following method steps: at least two base stations with known locations transmit a signal used in locating the terminal; the signals are transmitted in beams with directional antenna patterns; the direction of the beam is changed as a function of time; the terminal measures the signals transmitted by said at least two base stations and used in location determination, on the basis of which measurement the location of said base stations and the transmission direction of the signal used in location determination are determined and the location of the terminal is determined on the basis of the location of the base stations and the transmission direction of the signal used in location determination.
The arrangement of the invention is characterized in that at least two base stations with known locations comprise a transmitter for transmitting a signal used in locating the terminal and having a directed beam, and the transmitter is arranged to change the direction of the beam as a function of time; the terminal comprises means in a receiver which are arranged to measure the signals which are arranged to determine the location of the base station and the transmission angle of the signal used in location determination, and the radio system comprises means for determining the location of the terminal on the basis of the measurement performed by the terminal.
Several advantages can be achieved by the method and arrangement of the invention. The location of the terminal can be determined accurately with no need for great changes in the software of the base stations or terminals. The signalling load of the network part in the radio system is light. The arrangement of the invention is more accurate than the TA or OTD (Observed Time Difference) methods, for example, and no active call is necessary for determining the location. Furthermore, the arrangement of the invention operates equally well indoors and outdoors and the terminal can also determine its location itself. In addition, the solution according to the invention can easily be combined with other known locating solutions.